The distribution of calcium in mouse mandibular condyle was investigated by electron energy loss spectroscopy in order to determine the role of the matrix vesicles in cartilage mineralization. Ultrathin embedded cartilage sections from 1 week, 1 month, and 1 year old animals were analyzed both at the surface of the condyle and at different depths. High calcium concentrations were found inside vesicles near the mineralization but matrix vesicles close to the surface did not contain appreciable amounts of calcium either crystalline or amorphous. This suggests that the matrix vesicles near the surface may be involved in other processes, such as the fibrillation which occurs in non-inflammatory ulceration of cartilage or osteoarthritis.